I read somewhere that it is %20 slower to read from a computer screen than from paper. Is this true? I know for sure that I can read longer from a book because I start to get a headache from staring at the screen.

It's probably true, I'd take a book over a screen any day. Most of the decrease in speed is probably because of the strain on the eyes that most people get after prolonged periods of time in front of the screen.